Austenitic heat resisting steel

ABSTRACT

Cr-Ni type austenitic heat resisting steel having about twice the heat resistibility of common stainless steel by the addition of 0.001% to 0.30% by weight Ti, 0.001% to 0.30% by weight Nb + Ta and 0.05% to 2.50% by weight V to a well-known Cr-Ni type steel composition for elevated temperature service.

United; States Patent inventors" 'lohru Mlmlno;

Kuzuhlu Klnoshltu; 'lnkayukl Shlnoda; lsno Mlneglshl, all ofKawnsakl-shl, Japan Appl. No. 773,788

Filed Nov. 6, 1968 Patented Sept. 21,1971

Assignee Nippon Kokan Kabushiki Kaisha Priority Nov. 10, 1967 JapanAUSTENITIC HEAT RESISTING STEEL 1 Claim, 1 Drawing Fig.

US. Cl

75/128 T, 75/128 V Int. Cl C220 39/20 so1 FieldolSeurch 75/l2ii.6,128.8, 211.115

561 References Cited UNITED STATES PATENTS 2,190,486 2/1940 Schatmeister.Q. 75/l28.6 2,793,113 5/1957 Rait 75/12s.ss

Primary Examinerl-lyland Bizot Attorney-Flynn & Frishauf ABSTRACT: Cr-Nitype austenitic heat resisting steel having about twice the heatresistibility of common stainless steel by the addition of 0.001% to0.30% by weight Ti, 0.001% to 0.30% by weight Nb Ta and 0.05% to 2.50%by weight V to a well-known Cr-Ni type steel composition for elevatedtemperature service.

SUS 27 SUS 32 NO. I T

mama] mm mm c1607; 239

sus 2r su's 32 no. I

no a

CREEP RUTIRE 51m Kg/mm AUSTENITIC HEAT RESISTING STEEL BACKGROUND OF THEINVENTION This invention relates to Ci-Ni type austenitic stainlesssteels of improved heat resistibility for elevated temperature service.

DESCRIPTION OF THE PRIOR ART it is well known that higher strength andoxidation resistibility of steel at higher temperatures are required insome fields, c.g. boiler industry. As the above boiler becomes larger insize and of the ultra'critical pressure type. higher strength steel hasbeen called for said service.

At present, .118 (Japanese Industrial Standards) SUS-27, SUS'29, SUS-32and the like (AlSl-304, 321 and 316 steels are similar standards) 18Cr-8 Ni austenitic stainless steels are generally employed for elevatedtemperature and high-pressure services. The fact is that said SUS-32steel (AISI-316 steel) among the above-mentioned steels is commonlyemployed in consideration of having superior strengthat high temperaturein spite of a high cost which is due to M of 2% to I 3% content. Thecost of SUS-27 steel (AISI-304) is relatively low, while itshigh-temperature strength is inferior to other stainless steel. SUS-29steel (AISI-32l steel) with the addition of Ti to said SUS-27 steelcomposition has higher heat resistibility than said SUS-27 steel. It is,however, found that the strength of said SUS-29 steel fails remarkablyin a long period of service such as 650 C. 100,000 hours. Consequently,the strength is of the same degree as that of said SUS-27 steel.

Thus, a heat-resisting steel being in cost and having a higher strengthis not yet available for industrial circles. Accordingly, thedevelopment of more economical heat-resisting steels is required today.

The present invention'has been developed in order to meet the aboverequirement. The heat-resisting resisting steel of this invention ischaracterized by the addition of 0.001% to 0.30% by weight Ti, 0.001% to0.30% by weight Nb+Ta, and 0.05% to 2.5% by weight V to compositions ofthe above Standards steels.

' OBJECTS OF THE INVENTION BRIEF DESCRIPTION OF THE DRAWING Additionalobjects of this invention will become apparent by the followingdescription reference being made to the examples and the accompanyirigdrawing in which:

The FIGURE is a graph showing the relation of the creep rupture strengthin the case where steels subjected to elevated temperature service areJlS SUS-27 (AISI-304) and SUS-32 (AISI-3 l6) steels, this inventionsteels No. 1, No. 2, No. 3 and No. 4 and No. 5 steel not containing V.

DETAILED DESCRIPTION OF THE INVENTION According to the presentinvention, the steels of this invention have the following composition,that is, the steels consist of (by weight percent) 0.03% to 0.30% C, upto 1.00% Si, up to 2.00% Mn, 15.0% to 26.0% Cr, 7.0% to 22.0% Ni, 0.001%to 0.30% Ti, 0.001% to 0.30% Nb+Ta, 0.05% to 2.5% balance being Fe andunavoidable impurities.

As mentioned above, the reason for making said additional elements, thatis Ti, Nb+Ta and coexist lies in the fact that V, the

resting the agglomeration of those carbides produced by the aboveadditional elements and in making carbides disperse uniformly and finelyby interaction of said additional elements.

C content of the present invention steel is higher in value in order tocause Ti-carbide andNb-carbide to produce and then cause Cr-carbide andV-carbide to precipitate and disperse around said Ti and Nb-carbide.Consequently it causes the strength under elevated temperature serviceto improve.

The content of Ti should be defined to be within the above range. Morethan 0.30% Ti causes its carbide to coarsen and causes its strength tolower at high temperatures, while less than 0.001% 'li brings about alowering of precipitation amount of Ti-c'arbide and is ineffective forimproving its strength.

An effect of Nb+Ta, too, is similar to that of Ti. That is, both morethan 0.30% Ti and less than 0.001% Ti are impossible to causehigh-temperature strength to improve.

Less than 0.05% V is ineffective for improving its strength .at hightemperatures by reason of which the amount of V-carbide precipitated isdecreased, while more than 2.50% V :brings about a lowering ofhigh-temperature strength and causes its oxidation-resisting ability byreason of which the ,amount of V-carbide precipitatedrcmarkablyi'increases and [The compositions of SUS-27 stool (A151 -30!stool) 111111 51's BL'Slm-l (AlSI-316 steel) are based 011 L Standardl(. ])tt'1l\tl By weight [)tlt'lllt v C Si M11 C1- Ni Ti Nb+Ta Notes: 1.All compositions of No. 1,.No. 2, No. 3 and No. 4 steels are within therange defined as the present invention steel.

2. The composition of No. 5 steel is within the range as defined as thisinvention steel, exception being for V.

Having manufactured through the well-known process, said heat-resistingsteels of the aboveseven compositions were subjected to the creeprupture test under 600 C., 650 C. and 700 C. in 10 hr. and '10 hr.respectively. The results are shown in the following Table 2.

TABLE 2 Killogt'amsinnifi 000 0., hour 050 c., 11001 000 (1.11001 33. 027. 5 22. 5 17. 5 1-1. 11 12 z 33. a 29. 0 23.1 18. 1 10. 3 13. s 30.025.0 20.7 10.11 11.2 11.5 -1 30.2 25.11 21.11 111.0 in 12.3 5 23.0 10.510.0 12.5 11.2 a:

As mentioned above, the only difference of those steels, No. l to No. 4and No. 5 in composition was V. Referring now to the above Table 2 andFIG. I, it is clearly indicated that there is a great difference ofthose creep rupture test values. That is, it should be understood thatthe existence of V has an importanteffect upon the creep rupturestrength. Even No. 5.steel attained a higher strength, as compared withsaid SUS-27 and 32 steels by reason of which Ti and Nb+Ta is added tosaid SUS steels. This fact evidently explains that the addition of Ti,Nb+Ta and V causes the high-temperature strength to improve to a greatdegree. The creep rupture test value of this invention is more thantwice in comparison with that of said SUS -27 steel and is about twiceas much compared to said SUS-32 steel.

C 0.03 to 0.30 Si up 1.00 Mn up to 2.00 Cr l5.0 26.0 Ni 7.0 to 22 Ti0.00l to 0.30 Nb+Ta 0.00] to 0.30 V 0.05 to 2.5

balance Fe and unavoidable impurities

